there's a very strong feeling that this was a part of the story; that a group of people from this group, al qaeda, put innocent people in a position to have to do that. >> stahl: when you think about what terrorism means, this really says it. >> alice greenwald: absolutely. it's an impossible thing for a human being to do to another human being, and yet it became possible on 9/11. so, for us not to acknowledge that would be to not be true to the story. >> stahl: but how? with video of people falling, or photographs? and what about the feelings of family members? greenwald told us that she knows that some will never want to see an exhibit on this subject, but many argued strongly that it had to be there. >> alice greenwald: i have to say that we were also... i... i don't want to say accosted-- that's a little strong-- but, you know, shaken by the lapels by family members who said, "you have to tell the story. don't whitewash the story. tell it like it was. the world needs to know." >> joe daniels: so, we ultimately decided that we will include an exhibit, but do it in a way, in an alcov wher